This invention relates to an image processing apparatus and, more particularly, to an image processing apparatus for printing an output image on a prescribed recording medium based upon externally inputted print data.
A laser-beam printer is a typical example of an apparatus of this kind. Recent laser-beam printers are widely employed as computer output units. In particular, small-size laser-beam printers having a resolution on the order of 300 dpi (dots per inch) are rapidly becoming more popular owing to their low cost and compact nature.
As illustrated in FIG. 15, an exemplary 300 dpi laser-beam printer comprises a printer engine 200 for actually performing printing on a photosensitive drum based upon dot data, and a printer controller 100 connected to the printer engine 200 for receiving code data sent from an external host computer 300, generating page information consisting of dot data (bit-map data) based upon the code data, and transmitting the dot data to the printer engine 200 in successive fashion. The host computer 300 functions as a word processor, for example, for loading application software (an application program) from a floppy disk 500 having the application software, and then starting the application software.
The application software that can be used is of a large variety of types. Accordingly, the user keeps in his or her possession a large quantity of data created using this variety of application software.
The course of the printing operation in the printer controller 100 will now be described with reference to FIG. 18.
As shown in FIG. 18, the printer controller 100 includes an image memory 114 for storing one page of bit-map data (image data), an address generator 115 for generating addresses of the image memory 114, an output buffer register 116 for converting image data read out of the image memory 114 into an image signal VIDEO, a synchronizing clock generator circuit 117 for generating an image clock signal VCLK synchronized to a well-known beam-detection signal (BD signal), which is a horizontal synchronizing signal, a CPU 118 for administering overall control of the controller 100, a printer I/F, which is an input/output unit for interfacing signals with the printer engine 200, and a host I/F 1200, which is an input/output unit for interfacing signals with an external host, such as a personal computer.
Operation when the image signal VIDEO is sent to the printer engine in the above arrangement will now be described.
When one page of image data has been prepared in the image memory 114, the printer controller 100 sends a print-request signal PRINT to the printer engine 200. Upon receiving the PRINT signal, the printer engine 200 starts a printing operation and sends a VSREQ signal to the printer controller 100 at such time that a vertical synchronizing signal VSYNC (described later) is received and a state is attained in which printing can actually be carried out. When the VSREQ signal is received, the printer controller 100 sends the VSYNC signal to the printer engine 200 and, in order that printing may be carried out from a prescribed position in the subordinate scanning direction, counts a predetermined period of time from the moment of the VSYNC signal. When the counting of this predetermined period of time ends, the address generator 115 generates addresses successively from the beginning address of the image data stored in the image memory 114 and reads out the image data. The read image data is inputted to the output buffer register 116 line by line in terms of the main scanning lines. In order that printing may be performed from a prescribed position in the main scanning direction, a predetermined number of pulses in the image clock signal VCLK are counted after the BD signal, which is generated whenever a printing line is subjected to printing processing, is applied, after which the output buffer register 116 transmits the data of the prevailing printing line to the printer engine 200 as the image signal VIDEO synchronized to the VCLK signal. An image forming operation is then performed by the printer engine 200.
By performing the above-described operation for every of printing page, printing can be carried out on paper at the same position at all times.
As an example of actual operation, assume that a code corresponding to "a" (an ASCII code $61, where "$" indicates a hexadecimal number) has arrived from the host computer 300. In such case, the printer controller 100 develops a bit map and transfers the bit map to the printer engine 200 line by line so that the printer engine 200 performs printing as shown in FIG. 16.
However, since a character is composed of a collection of dots, the jaggedness of the character contour becomes conspicuous at a character density of 300 dpi. For this reason, printing using a printer having a higher resolution of, say, 600 dpi has been considered. However, when printing is merely performed at a recording density of 600 dpi using the dot composition as is, the size of the printed character is halved in both the vertical and horizontal direction.
More specifically, when consideration is given to the fact that most of the application software used thus far as well as the data created employing this application software has been produced for a resolution of 240.about.300 dpi, a problem which arises is that even if a 600 dpi printer is produced, it cannot cope with such application software. Accordingly, a technique is required through which the aforementioned application software can be exploited effectively while raising the definition of the printed output.
A method is available in which the dot composition is simply doubled within the printer in both the vertical and horizontal directions of the image data to be printed, whereby the 300 dpi dot composition is made to apply to 600 dpi. In accordance with this method, there is no reduction in the size of the printed character. However, when a character printed at 300 dpi and a character printed at 600 dpi are compared, no improvement whatsoever is seen in the jaggedness of the character contour. In short, the printing of characters cannot be carried out at a high quality which demonstrates the capability of a 600 dpi printer.
One example of a method proposed to solve this problem involves keeping the source data at 300 dpi, performing interpolation processing when the conversion from 300 dpi to 600 dpi is made using a 600 dpi printer engine, thereby improving upon the jagged edge of the contour, and printing at 600 dpi. With this method, however, it is required that the diameter of the laser beam in the printer engine be narrowed down, and the cost is very high.
Though proposals for improving upon the foregoing have been disclosed in the specifications of Japanese Patent Application Nos. 63-211501 and 63-211909 (the assignee of which is the that of the present application), which counterpart has been filed in USPTO as U.S. Ser. No. 07/398,822. These inventions involve printing in which a pixel of interest to be printed is subjected to smoothing signal processing by referring to data indicative of the surrounding or peripheral pixels and using a single prescribed algorithm.
Further, as set forth in the specification of Japanese Patent Application No. 1-224690, a method has been proposed in which the 300 dpi of the printer engine is maintained and only the recording density in the main scanning direction is raised to improve resolution [i.e., to achieve a resolution of, say, 1200 dpi (main scanning).times.300 dpi (subordinate scanning)] so that vertical lines in particular are printed smoothly.
In all of the foregoing methods, however, only a single prescribed algorithm can be applied.
In addition, smoothing signal processing of the above-mentioned kind is such that different data is produced in pseudo fashion. This means that it is essentially almost impossible to select, in alternative fashion, an optimum algorithm for all kinds of data, such as data indicative of characters, Figures and pictures (dither images), etc.
For example, the following problems arise:
(1) There is a difference between the effectiveness of smoothing applied to characters (black characters) and the effectiveness of smoothing applied to reversed characters (white characters).
(2) Even if a character is capable of being smoothed, an attendant drawback is that the density of a dither image is raised, thereby resulting in a darkened picture.
(3) Characters of small size are deformed.
(4) Preferences regarding the effects of character smoothing differ depending upon the individual.
The prior is incapable of solving problems of this kind.